The present invention is directed to a method of making aluminum sheet product and products therefrom and, in particular, to a method of controlling hot rolling entry temperatures and hot rolling exit temperatures during automotive sheet processing to produce a high strength final gauge product in which certain surface defects, referred to as ridging, have been minimized or eliminated.
In the automotive industry, the use of aluminum alloys for automotive applications is increasing due to their desirable combination of properties such as low density, high strength, corrosion resistance, and formability.
Typically, automotive body sheet products are approximately 0.04xe2x80x3 (1.0 mm) in thickness and include 2000 and 6000 series aluminum alloys. Preferred 2000 series aluminum alloys include, but are not limited to, AA2008, AA2010, and AA2036. Preferred 6000 series aluminum alloys include, but are not limited to, AA6009, AA6010, AA6016, and AA6111. Outer body panel applications require high strength, typically to improve dent resistance, and superior surface appearance, to ensure that no discontinuities are visible after paint has been applied. Inner body panel applications do not require the strength levels or surface appearance necessary for outer body panel applications; however, formability is a critical concern because of the complex nature of most inner body panel designs.
With reference to FIG. 1, a typical prior art processing sequence is depicted for making aluminum sheet. Ingots of the aluminum alloy are cast, homogenized and hot rolled to a desired gauge, such as approximately 0.18xe2x80x3 (4.6 mm). Typically, the homogenized ingot enters the hot rolling step at a temperature of approximately 850xc2x0F. (454xc2x0 C.)to 1000xc2x0 F. (538xc2x0 C.) and exits at temperatures generally greater than 650xc2x0 F. (343xc2x0 C.). The hot rolled material is then cold rolled to final gauge, solution heat treated, leveled, and formed for its desired end use. The material may be subjected to an anneal either prior to the cold rolling or after completion of a portion of the cold rolling. U.S. Pat. No. 4,614,552, titled ALUMINUM ALLOY SHEET PRODUCT, the contents of which are herein incorporated by reference, describes conventional prior art practice for producing AA6111 alloys. U.S. patent application Ser. No. 08/246,653, filed May 20, 1994, titled METHOD OF MAKING ALUMINUM SHEET PRODUCT AND PRODUCT THEREFROM and Japanese Application No. 6-41850 of Feb. 16, 1994, titled METHOD OF PRODUCING ALUMINUM ALLOY SHEET HAVING HIGH FORMABILITY.
One drawback associated with the use of aluminum alloys for automotive components is the possible presence of objectionable and/or deleterious surface defects referred to as ridging, roping, or paint brush lines, which appear on the surface of stamped or formed aluminum sheet components. The ridging lines are present on the surface of the automotive component as a series of closely spaced lines in the rolling direction. Typically, the ridging lines are approximately 0.2 microns high and are spaced less than approximately 1 mm apart. The ridging or roping lines appear in the rolling direction only upon application of sufficient transverse strain, such as that occurring in typical stamping or forming operations.
This ridging defect when present is of sufficient severity to be visible in the automotive component after painting. Consequently, the finished surface appearance of components made from these aluminum alloys is objectionable and not suitable for exterior automotive applications. This ridging defect may also serve as a strain concentration site during forming, thus limiting formability.
In view of the problems with automotive components processed using aluminum sheet made with prior art practices, a need has developed to eliminate the ridging line defects in aluminum alloy sheet products. Elimination of this defect should encourage increased application of aluminum alloys.
In response to the preceding, the present invention provides an improved sheet product and a method of making aluminum alloy sheet product, especially for automotive use, which minimizes or eliminates the occurrence of ridging lines while maintaining the potential for high strength in the naturally aged and artificially aged conditions. The inventive method produces an aluminum alloy sheet product which can be stamped or formed and painted for automotive use without an objectionable surface appearance having defects, such as ridging or roping lines.
Accordingly, a first object of the present invention is to provide a method of making aluminum alloy sheet product which eliminates or minimizes ridging lines in the finished product without compromising the strength of the finished product.
Another objective of the present invention is to provide a method of making aluminum alloy sheet product which also improves the formability of the sheet for subsequent forming and/or stamping.
Other objects and advantages of the present invention will become apparent as a description thereof proceeds.
In satisfaction of the foregoing objects and advantages, the present invention provides an improvement over the prior art method of producing aluminum alloy sheet product, especially for automotive use, wherein the aluminum alloy is cast, homogenized, hot rolled to an intermediate gauge, cold rolled to final gauge, and solution heat treated for subsequent automotive component use. It should be appreciated that reference to the term xe2x80x9cautomotivexe2x80x9d is intended to encompass similar applications, such as for light truck component use.
According to the invention, the temperature of the hot rolling of the aluminum alloy and other process parameters are controlled so that the temperature of the ingot entering the hot mill is greater than 925xc2x0 F. (496xc2x0 C.) and the temperature of the aluminum alloy exiting the rolling mill is less than about 600xc2x0 F. (316xc2x0 C.). The hot rolled product is then cold rolled to final gauge, with the application of an intermediate anneal following cold rolling of the hot line product between 0% and 70% of the total cold rolling reduction. In one embodiment, it is preferred that the temperature of the ingot upon entering the hot mill be greater than 950xc2x0 F. (510xc2x0 C.) and the temperature of the intermediate product exiting the hot mill not exceed 575xc2x0 F. (302xc2x0 C.).
In the embodiment wherein the sheet is annealed after 0% cold work, a preferred annealing temperature ranges between 700xc2x0 F. (371xc2x0 C.) and 800xc2x0 F. (427xc2x0 C.) for about two hours. In the alternative embodiment wherein the intermediate gauge sheet is annealed after about 45% to 70% cold work, a preferred annealing temperature range is between 600xc2x0 F. (316xc2x0 C.) and 700xc2x0 F. (371xc2x0 C.) for about two hours.
Preferably, the aluminum alloy used in the inventive process is either an AA6000 series or an AA2000 series alloy, more preferably, an AA6111 type alloy. However, any aluminum alloy adaptable for sheet use can be utilized in the invention.
The inventive method produces an intermediate product in the form of a hot rolled band which can be subsequently annealed and cold rolled to a final gauge sheet product devoid of or having a minimum of ridging or roping lines on the surface thereof. The invention also provides a final cold rolled gauge sheet product which can be solution heat treated, leveled, and formed as an automotive component.